Developed additive enables low-cost storage of renewable energy

Solar and wind are rapidly changing the energy landscape, but if we want to realize the full potential of these intermittent renewable energy sources, we will need safe and affordable batteries that can store it.

As part of an effort to solve the long-term problem of energy storage, engineers at the University of Wisconsin-Madison have invented a water-soluble chemical. additive which improves the performance of an electrochemical storage device called bromide aqueous solution. flow battery.

“Bromide-based aqueous flow batteries are a promising solution, but they pose many complex electrochemical challenges. That’s why there are no truly successful bromide products today,” says Patrick Sullivan, who graduated from UW-Madison with a Ph.D. in Chemistry in 2023. “However, our one supplement can solve many different problems.”

Sullivan, Ph.D. student Gyohun Choi and Dawei Feng, associate professor of the department materials science and engineers from the University of Wisconsin at Madison developed the additive. The study was published October 23, 2024, magazine Nature.

Currently, giant lithium-ion batteries the size of tractor-trailers store energy for the grid, but with technical limitations. Lithium batteries raise safety concerns due to the potential for fire and explosion and the complex international supply chain.

However, aqueous flow batteries could make network-attached storage safer and cheaper. In these batteries, positive and negative liquid electrolytes circulate through electrodes separated by a membrane. Because batteries use ions dissolved in a liquid—water—they can be scalable, sustainable, and safe.

Most commercially developed flow batteries are based on vanadium ions, which, like lithium, are expensive and difficult to obtain. However, another version of these flow batteries is based on bromide, a cheap and widely available ion that has similar characteristics to vanadium—at least on paper.

However, in practice, tiny bromide ions cause all sorts of problems in flow batteries. They can pass through the membrane separating the electrodes, and this reduces the battery’s efficiency. Sometimes ions precipitate out of the electrolyte and form a dirty oil that “sinks” to the bottom of the solution. Sometimes the ions also form toxic bromine gas. These issues reduce practical performance and reliability.

An additive called a complexing agent can help with this. Choi set out to find an additive that improves the performance of bromide water flow batteries. Researchers used molecular design to develop more than 500 candidates organic molecules they call “soft-hard zwitterion traps.” They synthesized and tested 13 of these representative molecules as potential additives for bromide batteries.

The resulting multifunctional additives solve the main problems of flow batteries. It encapsulates the bromide ions, allowing them to remain water soluble, and since the complex formed is now larger, they cannot pass through the membrane. The ions are also “phase stable,” meaning they do not separate from the aqueous electrolyte and create toxic bromine gas.

It is important to note that additives significantly improve the performance of a flow battery, increasing the efficiency and longevity of the chemical system. “Our devices with the additive worked without failure for almost two months, compared to devices without it, which usually fail within a day,” says Feng. “This is important because to store clean energy, you want to use it for 10 or 20 years.”

The team plans to continue refining the work. Choi will study the basic science behind additives for bromide and iodide flow batteries, and Sullivan, who is CEO of Flux XII—the renewable energy subsidiary he co-founded with Feng—will study the commercial viability of an additive that has already has been researched. successfully produced in industrial ton-scale reactions.